Method for generating downlink frame, and method for searching cell

ABSTRACT

The present invention relates to a method for generating a downlink frame includes: generating a first short sequence and a second short sequence indicating cell group information; generating a first scrambling sequence determined by a first synchronization signal; generating a second scrambling sequence determined by a first short sequence and a third scrambling sequence determined by a second short sequence; scrambling the first short sequence with the first scrambling sequence; scrambling the second short sequence with at least the second scrambling sequence; scrambling the second short sequence with the first scrambling sequence; scrambling the first short sequence with at least the third scrambling sequence; and mapping one second synchronization signal including the first short sequence scrambled with the first scrambling sequence and the second short sequence scrambled with at least the second scrambling sequence and another second synchronization signal including the second short sequence scrambled with the first scrambling sequence and the first short sequence scrambled with at least the third scrambling sequence in the frequency domain.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of PCT application No.PCT/KR2008/004094, filed on Jul. 11, 2008, which claims priority to andthe benefit of Korean Patent Application No. 10-2007-0070086 filed onJul. 12, 2007, Korean Patent Application No. 10-2007-0082678 filed onAug. 17, 2007, Korean Patent Application No. 10-2007-0083916 filed onAug. 21, 2007, Korean Patent Application No. 10-2008-0064202 filed onJul. 3, 2008, the entire contents of which are incorporated herein byreference.

BACKGROUND

(a) Field

The present invention relates to a downlink frame generation method anda cell search method, and particularly relates to a method forgenerating a downlink frame and a method for searching a cell by usingthe downlink frame in an orthogonal frequency division multiplexing(OFDM)-based cellular system.

(b) Description of the Related Art

In a direct sequence code division multiple access (DS-CDMA) system, thecode hopping method is applied to a pilot channel in order to acquirecell synchronization and appropriate cell identification information.The code hopping method introduces a code hopping technique to the pilotchannel so that a terminal may easily search the cell without anadditional synchronization channel. However, since the number ofchannels that are distinguishable by the frequency domain in the symbolinterval is much greater than the number of channels that aredistinguishable by the CDMA spread in one time domain symbol interval inthe OFDM system, the usage of the time domain may waste resources withrespect to capacity, and hence it is difficult to apply the code hoppingmethod to the pilot channel time domain of the OFDM-based system.Therefore, it is desirable in the OFDM case to search cells byefficiently using the received signals in the time domain and thefrequency domain.

A conventional technique for searching cells in the OFDM system includesdivide a frame into four time blocks and allocating synchronizationinformation and cell information. The technique proposes two framestructures. The first frame structure allocates synchronizationrecognition information, cell group recognition information, appropriatecell recognition information, and synchronization recognitioninformation to four time blocks. The second frame structure allocatessynchronization recognition information and appropriate cell recognitioninformation to the first time block and the third time block, andsynchronization recognition information and cell group recognitioninformation to the second time block and the fourth time block.

In the case of following the first scheme, since symbol synchronizationis acquired in the first time block, it is impossible to acquire fastsynchronization within the standard of 5 ms when a terminal is turned onor in the case of a handover between heterogeneous networks. Also, it isdifficult to acquire a diversity gain through accumulation ofsynchronization recognition information for fast synchronizationacquisition.

In the case of following the second scheme, the cell search process iscomplicated and it is difficult to search the cells quickly since it isrequired to acquire synchronization and simultaneously correlateappropriate cell recognition information or cell group recognitioninformation so as to acquire frame synchronization.

Another method for searching cells using an additional preamble toacquire synchronization and search the cells has been proposed, but itis inapplicable to a system having no preamble. Further, since thepreamble is disposed at the front part of the frame, the terminal muststand by for the next frame when attempting to acquire synchronizationat a time position other than the first time position of the frame.Particularly, when the terminal performs a handover among the GSM mode,the WCDMA mode, and the 3GPP LTE mode, it must acquire the initialsymbol synchronization within 5 msec, but the initial symbolsynchronization may not be acquired within 5 msec since thesynchronization can be acquired for each frame.

SUMMARY

The present invention has been made in an effort to provide a downlinkframe generating method for averaging interference between sectors, andan efficient cell searching method by receiving the downlink frame.

An exemplary embodiment of the present invention provides a method forgenerating a downlink frame including a first synchronization signal anda second synchronization signal includes: generating a first shortsequence and a second short sequence for indicating cell groupinformation; generating a first scrambling sequence determined by thefirst synchronization signal; generating a second scrambling sequencedetermined by the first short sequence and a third scrambling sequencedetermined by the second short sequence; scrambling the first shortsequence with the first scrambling sequence, and scrambling the secondshort sequence with at least the second scrambling sequence; scramblingthe second short sequence with the first scrambling sequence, andscrambling the first short sequence with at least the third scramblingsequence; and mapping one second synchronization signal including thefirst short sequence scrambled with the first scrambling sequence andthe second short sequence scrambled with at least the second scramblingsequence and another second synchronization signal including the secondshort sequence scrambled with the first scrambling sequence and thefirst short sequence scrambled with at least the third scramblingsequence in the frequency domain.

Another embodiment of the present invention provides a device forgenerating a downlink frame including a first synchronization signal anda second synchronization signal includes: a sequence generator forgenerating a first short sequence and a second short sequence forindicating cell group information, a first scrambling sequencedetermined by the first synchronization signal, a second scramblingsequence determined by the first short sequence, and a third scramblingsequence determined by the second short sequence; and a synchronizationsignal generator for generating one second synchronization signal byscrambling the first short sequence with the first scrambling sequenceand scrambling the second short sequence with at least the secondscrambling sequence, and generating another second synchronizationsignal by scrambling the second short sequence with the first scramblingsequence and scrambling the first short sequence with at least the thirdscrambling sequence.

Yet another embodiment of the present invention provides a recordingmedium for recording a program for performing a method for generating adownlink frame including a first synchronization signal and a secondsynchronization signal includes: generating a first short sequence and asecond short sequence indicating cell group information; generating afirst scrambling sequence determined by the first synchronizationsignal; generating a second scrambling sequence determined by the firstshort sequence and a third scrambling sequence determined by the secondshort sequence; scrambling the first short sequence with the firstscrambling sequence, and scrambling the second short sequence with atleast the second scrambling sequence; scrambling the second shortsequence with the first scrambling sequence, and scrambling the firstshort sequence with at least the third scrambling sequence; and mappingone second synchronization signal including the first short sequencescrambled with the first scrambling sequence and the second shortsequence scrambled with at least the second scrambling sequence andanother second synchronization signal including the second shortsequence scrambled with the first scrambling sequence and the firstshort sequence scrambled with at least the third scrambling sequence inthe frequency domain.

According to the present invention, cell search performance is improvedby scrambling a short sequence with a scrambling sequence and reducinginterference between sectors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a downlink frame of an OFDM system according to anexemplary embodiment of the present invention.

FIG. 2 shows a synchronization channel configuration diagram indicatinga secondary synchronization channel when two sequences are mapped on thefrequency domain in a centralized manner.

FIG. 3 shows a synchronization channel configuration diagram indicatinga secondary synchronization channel when two sequences are mapped on thefrequency domain in a distributive manner.

FIG. 4 shows a block diagram of a downlink frame generating deviceaccording to an exemplary embodiment of the present invention.

FIG. 5 shows a flowchart of a downlink frame generating method accordingto an exemplary embodiment of the present invention.

FIG. 6 shows a method for generating a secondary synchronization signalaccording to a first exemplary embodiment of the present invention.

FIG. 7 shows a method for generating a secondary synchronization signalaccording to a second exemplary embodiment of the present invention.

FIG. 8 shows a block diagram of a cell searching device according to anexemplary embodiment of the present invention.

FIG. 9 shows a flowchart of a cell searching method according to a firstexemplary embodiment of the present invention.

FIG. 10 shows a flowchart of a cell searching method according to asecond exemplary embodiment of the present invention.

DETAILED DESCRIPTION

In the following detailed description, only certain exemplaryembodiments of the present invention have been shown and described,simply by way of illustration. As those skilled in the art wouldrealize, the described embodiments may be modified in various differentways, all without departing from the spirit or scope of the presentinvention. For clarification of drawings in the present invention, partsthat are not related to the description will be omitted, and the samepart will have the same reference sequence throughout the specification.

Throughout the specification, unless explicitly described to thecontrary, the word “comprise” and variations such as “comprises” or“comprising”, will be understood to imply the inclusion of statedelements but not the exclusion of any other elements. Also, the terms ofa unit, a device, and a module in the present specification represent aunit for processing a predetermined function or operation, which can berealized by hardware, software, or combination of hardware and software.

A downlink frame and a synchronization channel of an OFDM systemaccording to an exemplary embodiment of the present invention will nowbe described with reference to FIG. 1 to FIG. 3.

FIG. 1 shows a downlink frame of an OFDM system according to anexemplary embodiment of the present invention. In FIG. 1, the horizontalaxis represents the time axis, and the vertical axis represents thefrequency axis or a subcarrier axis.

As shown in FIG. 1, a downlink frame 110 according to an exemplaryembodiment of the present invention has a time interval of 10 msec andincludes ten subframes 120. One subframe 120 has a time interval of 1msec and includes two slots 130, and one slot 130 includes six or sevenOFDM symbols. When one slot includes six symbols, it has a cyclic prefixlength that is greater than the case in which one slot includes sevensymbols.

As shown in FIG. 1, one downlink frame 110 according to an exemplaryembodiment of the present invention includes one synchronizationinterval 140 at the slot 0 and the slot 10 respectively, to thus includetwo synchronization intervals 140. However, the embodiment of thepresent invention is not restricted to this. That is, one downlink frame110 may include a synchronization interval at a random slot, and mayinclude one or at least three synchronization intervals. Also, since thelength of the cyclic prefix may be different for each slot, it isdesirable to provide the synchronization interval to the last positionof the slot.

Each slot includes a pilot interval.

A synchronization interval according to an exemplary embodiment of thepresent invention includes a primary synchronization channel and asecondary synchronization channel, and the primary synchronizationchannel and the secondary synchronization channel are disposedadjacently with respect to time. As shown in FIG. 1, a primarysynchronization channel is provided to the last position of the slot,and a secondary synchronization channel is provided before the primarysynchronization channel.

The primary synchronization channel includes information for identifyingsymbol synchronization and frequency synchronization, and some cell ID(identification) information, and the secondary synchronization channelincludes information for identifying other cell ID information and framesynchronization. The mobile station identifies the cell ID throughcombination of cell ID information of the primary and secondarysynchronization channels.

For example, when there are 510 cell IDs, three primary synchronizationsignals are allocated to the primary synchronization channel to dividethe entire 510 cell IDs into three groups, and when 170 secondarysynchronization signals are allocated to the secondary synchronizationchannel, the entire 510 cell ID information (3×170=510) can beexpressed.

Further, it is also possible to divide the 510 cell IDs into 170 groupsby using the 170 secondary synchronization signals allocated to thesecondary synchronization channel, and express the cell ID informationin the cell groups by using the three primary synchronization signalsallocated to the primary synchronization channel.

Since the secondary synchronization channel includes information foridentifying frame synchronization as well as cell ID information, twosecondary synchronization channels included in one frame are different.

FIG. 2 shows a synchronization channel configuration diagram indicatinga secondary synchronization channel when two sequences are mapped on thefrequency domain in a centralized manner, and FIG. 3 shows asynchronization channel configuration diagram indicating a secondarysynchronization channel when two sequences are mapped on the frequencydomain in a distributive manner.

Referring to FIG. 2 to FIG. 3, a secondary synchronization signalinserted into a secondary synchronization channel according to anexemplary embodiment of the present invention is configured by acombination of two sequences. Cell group information and framesynchronization information are mapped on the two sequences.

As shown in FIG. 2, it is possible to allocate the first sequence to thesubcarrier and sequentially allocate the second sequence to the othersubcarrier, and as shown in FIG. 3, it is possible to allocate the firstsequence to every even subcarrier (n=0, 2, 4, and . . . 60) and thesecond sequence to every odd subcarrier (n=1, 3, 5, and . . . 61).

The length of sequence is half the number of subcarriers allocated tothe secondary synchronization channel. That is, the number of elementsof the sequence that can be generated is as many as half the number ofsubcarriers allocated to the secondary synchronization channel. Forexample, when the number of subcarriers allocated to the secondarysynchronization channel is 62, the length of the sequence is 31, and upto 31 elements of the sequence can be generated.

Therefore, since two sequences are allocated to one secondarysynchronization channel, 961 (=31×31) secondary synchronization signalsare generated. However, since the information to be included by thesecondary synchronization channel includes cell group information andframe boundary information, 170 or 340 (=170×2) secondarysynchronization signals are needed. That is, 961 is sufficiently greaterthan 170 or 340.

A downlink frame generating device according to an exemplary embodimentof the present invention will now be described with reference to FIG. 4.FIG. 4 shows a block diagram of a downlink frame generating deviceaccording to an exemplary embodiment of the present invention.

As shown in FIG. 4, the downlink frame generating device includes asequence generator 410, a synchronization signal generator 420, afrequency mapper 430, and an OFDM transmitter 440.

The sequence generator 410 generates a time and frequencysynchronization acquiring sequence, a cell identifying sequence, aplurality of short sequences, and an adjacent cell interference reducingscrambling sequence, and transmits them to the synchronization signalgenerator 420.

The synchronization signal generator 420 generates a primarysynchronization signal, a secondary synchronization signal, and a pilotpattern by using the sequences transmitted by the sequence generator410.

The synchronization signal generator 420 generates a primarysynchronization signal by using a time and frequency synchronizationacquiring sequence and a cell identifying sequence. The synchronizationsignal generator 420 generates a secondary synchronization signal byusing a plurality of short sequences and an adjacent cell interferencereducing scrambling sequence.

The synchronization signal generator 420 generates a pilot pattern of adownlink signal by allocating a proper scrambling sequence that isallocated for each cell to the pilot channel so as to encode a commonpilot symbol and a data symbol of the cellular system.

The frequency mapper 430 maps the primary synchronization signal, thesecondary synchronization signal, and the pilot pattern generated by thesynchronization signal generator 420, and frame control information andtransmission traffic data provided from the outside in the time andfrequency domains to generate a downlink frame.

The OFDM transmitter 440 receives the downlink frame from the frequencymapper 430 and transmits it through a transmitting antenna.

A downlink frame generating method according to an exemplary embodimentof the present invention will now be described with reference to FIG. 5to FIG. 7. FIG. 5 shows a flowchart of a downlink frame generatingmethod according to an exemplary embodiment of the present invention.

As shown in FIG. 5, the sequence generator 410 generates a plurality ofshort sequences and a plurality of adjacent cell interference reducingscrambling sequences and transmits them to the synchronization signalgenerator 420 (S510).

The synchronization signal generator 420 generates a secondarysynchronization signal by using the short sequences and the adjacentcell interference reducing scrambling sequences transmitted by thesequence generator 410 (S520). The exemplary embodiment of the presentinvention will exemplify the frame including two secondarysynchronization channels, but is not limited thereto.

Two secondary synchronization signal generating methods according to anexemplary embodiment of the present invention will now be described withreference to FIG. 6 and FIG. 7. FIG. 6 shows a first method forgenerating a secondary synchronization signal according to an exemplaryembodiment of the present invention, and FIG. 7 shows a second methodfor generating a secondary synchronization signal according to anexemplary embodiment of the present invention.

A short sequence (wn) is a binary sequence (binary code) indicating cellgroup information. That is, the short sequence (wn) is a binary sequenceallocated for the cell group number and the frame synchronization, andits length is half the number of subcarriers allocated to the secondarysynchronization channel. The exemplary embodiment of the presentinvention describes the case in which the number of subcarriersallocated to the secondary synchronization channel symbol is 62, but isnot limited thereto. Therefore, the length of the short sequenceaccording to the exemplary embodiment of the present invention is 31.

The first short sequence (w0) is a sequence allocated to the evensubcarrier of the first (slot 0) secondary synchronization channel andis expressed in Equation 1.w0=[w0(0), w0(1), . . . , w0(k), . . . , w0(30)]  [Equation 1]

Here, k represents an index of the even subcarrier used for thesynchronization channel.

The second short sequence (w1) is a sequence allocated to the oddsubcarrier of the first (slot 0) secondary synchronization channel andis expressed in Equation 2.w1=[w1(0), w1(1), . . . , w1(m), . . . w1(30)]  [Equation 2]

Here, m represents an index of the odd subcarrier used for thesynchronization channel.

The third short sequence (w2) is a sequence allocated to the evensubcarrier of the second (slot 10) secondary synchronization channel andis expressed in Equation 3.w2=[w2(0), w2(1), . . . , w2(k), . . . , w2(30)]  [Equation 3]

The fourth short sequence (w3) is a sequence allocated to the oddsubcarrier of the second (slot 10) secondary synchronization channel andis expressed in Equation 4.w3=[w3(0), w3(1), . . . , w3(m), . . . w3(30)]  [Equation 4]

w0, w1, w2, and w3 may be different sequences from each other, and itmay be that w0=w3 and w1=w2, or it may be that w0=w2 and w1=w3. When itis given that w0=w3 and w1=w2, the short sequences of the secondsecondary synchronization channel can be allocated by using the shortsequences allocated to the first synchronization channel, and a terminalonly needs to memorize the 170 short sequences allocated to the firstsecondary synchronization channel and thereby reduce the complexity.

The first method for generating the secondary synchronization signal isto allocate the first short sequence to every even subcarrier of thefirst secondary synchronization channel and the second short sequence toevery odd subcarrier of the first secondary synchronization channel asshown in FIG. 6. The first method is then to allocate the third shortsequence to every even subcarrier of the second secondarysynchronization channel and the fourth short sequence to every oddsubcarrier of the second secondary synchronization channel.

According to the first method for generating the secondarysynchronization signal, since the secondary synchronization signal isgenerated by the combination of two short sequences with the length of31, the number of the secondary synchronization signals becomes 961which is sufficiently greater than the required number of 170 or 340.

The second method for generating the secondary synchronization signal isto allocate the first sequence determined by Equation 5 to every evensubcarrier of the first (slot 0) secondary synchronization channel, andthe second sequence determined by Equation 6 to every odd subcarrier ofthe first (slot 0) secondary synchronization channel, as shown in FIG.7. The second method also includes allocating the third sequencedetermined by Equation 7 to every even subcarrier of the second (slot10) secondary synchronization channel, and the fourth sequencedetermined by Equation 8 to every odd subcarrier of the second (slot 10)secondary synchronization channel.

A scrambling sequence P_(j,1) for scrambling the first short sequence w0is given as P_(j,1)=[P_(j,1)(0), P_(j,1)(1), . . . P_(j,1)(k) . . .P_(j,1)(30)], and j (j=0, 1, 2) is a number of a cell identifyingsequence allocated to the primary synchronization channel. Therefore,P_(j,1) is determined by the primary synchronization signal. P_(j,1) isa known value when the mobile station demaps the sequence in order toknow the cell ID group and the frame boundary.

As expressed in Equation 5, respective elements of the first sequence c₀according to the second method for generating the secondarysynchronization signal are products of respective elements of the firstshort sequence w0 and respective corresponding elements of P_(j,1).c ₀ =[w0(0)P _(j,1)(0), w0(1)P _(j,1)(1), . . . , w0(k)P _(j,1)(k), . .. , w0(30)P _(j,1)(30)]  [Equation 5]

Here, k is an index of the even subcarrier used for the synchronizationchannel.

A scrambling sequence S_(w0) for scrambling the second short sequence w1is given as S_(w0)=[S_(w0)(0), S_(w0)(1), . . . , S_(w0)(m), . . . ,S_(w0)(30)], and S_(w0) is determined by the first short sequence (w0).

In this instance, it is possible to determine S_(w0) according to theshort sequence group to which the first short sequence belongs bycombining the short sequences into a group.

For example, since the length of the short sequence is 31 in theexemplary embodiment of the present invention, there are 31 shortsequences. Therefore, the 0 to 7 short sequences are set to belong tothe group 0, the 8 to 15 short sequences are set to belong to the group1, the 16 to 23 short sequences are set to belong to the group 2, andthe 24 to 30 short sequences are set to belong to the group 3, ascrambling code is mapped on each group, and the scrambling code mappedon the group to which the first short sequence belongs is determined tobe S_(w0).

It is possible to divide the number of the short sequence by 8, combinethe short sequences having the same residuals, and thereby classify the31 short sequences as 8 groups. That is, the number of the shortsequence is divided by 8, the short sequence having the residual 0 isset to belong to the group 0, the short sequence having the residual 1is set to belong to the group 1, the short sequence having the residual2 is set to belong to the group 2, the short sequence having theresidual 3 is set to belong to the group 3, the short sequence havingthe residual 4 is set to belong to the group 4, the short sequencehaving the residual 5 is set to belong to the group 5, the shortsequence having the residual 6 is set to belong to the group 6, theshort sequence having the residual 7 is set to belong to the group 7, ascrambling code is mapped on each group, and the scrambling code mappedon the group to which the first short sequence belongs is determined tobe S_(w0).

As expressed in Equation 6, the respective elements of the secondsequence c₁ according to the second method for generating the secondarysynchronization signal are products of the respective elements of thesecond short sequence w1 and the corresponding respective elements ofS_(w0).c ₁ =[w ₁(0)S _(w0)(0), w1(1)S _(w0)(1), . . . , w1(m)S _(w0() m), . . ., w1(30)S _(w0)(30)]  [Equation 6]

Here, m is an index of the odd subcarrier used for the synchronizationchannel.

The scrambling sequence P_(j,2) for scrambling the third short sequencew2 is given as P_(j,2)=[P_(j,2)(0), P_(j,2)(1), . . . P_(j,2)(k) . . .P_(j,2)(30)], and j (j=0, 1, 2) is a number of a cell identifyingsequence allocated to the primary synchronization channel. Therefore,P_(j,2) is determined by the primary synchronization signal. P_(j,2) isa known value when the terminal demaps the code in order to know thecell ID group and the frame boundary.

As expressed in Equation 7, the respective elements of the thirdsequence c₂ according to the second method for generating the secondarysynchronization signal are products of the respective elements of thethird short sequence w2 and the corresponding respective elements ofP_(j,2).c ₂ =[w2(0)P _(j,2)(0), w2(1)P _(j,2)(1), . . . , w2(k)P _(j,2)(k), . .. , w2(30)P _(j,2)(30)]  [Equation 7]

Here, k is an index of the even subcarrier used for the synchronizationchannel.

The scrambling sequence S_(w2) for scrambling the fourth short sequenceis given as S_(w2)=[S_(w2)(0), S_(w2)(1), S_(w2)(m), . . . S_(w2)(30)],and S_(w2) is determined by the third short sequence w2.

In this instance, it is possible to combine the short sequences into agroup and determine S_(w2) according to the short sequence group towhich the third short sequence belongs.

For example, since the length of the short sequence according to theexemplary embodiment of the present invention is 31, there are 31 shortsequences. Therefore, the 0 to 7 short sequences are set to belong tothe group 0, the 8 to 15 short sequences are set to belong to the group1, the 16 to 23 short sequences are set to belong to the group 2, the 24to 30 short sequences are set to belong to the group 3, a scramblingcode is mapped on each group, and the scrambling code mapped on thegroup to which the third short sequence belongs is determine to beS_(w2).

It is also possible to divide the number of the short sequence by 8,combine the short sequences with the same residual, and classify the 31short sequences as 8 groups. That is, the number of the short sequenceis divided by 8, the short sequence with the residual 0 is set to belongto the group 0, the short sequence with the residual 1 is set to belongto the group 1, the short sequence with the residual 2 is set to belongto the group 2, the short sequence with the residual 3 is set to belongto the group 3, the short sequence with the residual 4 is set to belongto the group 4, the short sequence with the residual 5 is set to belongto the group 5, the short sequence with the residual 6 is set to belongto the group 6, the short sequence with the residual 7 is set to belongto the group 7, a scrambling code is mapped on each group, and thescrambling code mapped on the group to which the third short sequencebelongs is determined to be S_(w2).

As expressed in Equation 8, the respective elements of the fourthsequence c₃ according to the second method for generating the secondarysynchronization signal are the products of the respective elements ofthe fourth short sequence and the corresponding respective elements ofS_(w2).c ₃ =[w3(0)S _(w2) 0, w3(1)S _(w2)(1), . . . , w3(m)S _(w2)(m), . . .w3(30)S _(w2)(30)]  [Equation 8]

Here, m is an index of the odd subcarrier used for the synchronizationchannel.

Here, it is given that P_(j,1)=P_(j,2), and w0≠w1≠w2≠w3 or w0=w3, w1=w2.In this case, the cell group and frame identifying information aremapped on the combination of the first to fourth short sequences, andthe number of descrambling hypotheses of the terminal for the scrambleof the secondary synchronization channel defined by the cell identifyingsequence number of the primary synchronization channel is reduced.

It is set that P_(j,1)≠P_(j,2) and w0=w2, w1=w3. In this case, cellgroup information is mapped on the combination of the first shortsequence and the second short sequence, and frame synchronizationinformation is mapped on the scrambling sequences P_(j,1) and P_(j,2) ofthe secondary synchronization channel defined by the cell identifyingsequence number of the primary synchronization channel. The number ofdescrambling hypotheses of the terminal for the scramble of thesecondary synchronization channel defined by the cell identifyingsequence number of the primary synchronization channel is increased, butthe complexity is reduced since the combination of cell groupidentifying sequences is reduced to half.

The frequency mapper 430 maps the secondary synchronization signal andthe transmission traffic data generated by the synchronization signalgenerator 420 in the time and frequency domains to generate a frame ofthe downlink signal (S530).

The OFDM transmitter 440 receives the frame of the downlink signal andtransmits it through the transmitting antenna (S540).

A method for a terminal to search the cell by using a downlink signalaccording to an exemplary embodiment of the present invention will nowbe described with reference to FIG. 8 to FIG. 10.

FIG. 8 shows a block diagram of a cell searching device according to anexemplary embodiment of the present invention, FIG. 9 shows a flowchartof a cell searching method according to a first exemplary embodiment ofthe present invention, and FIG. 10 shows a flowchart of a cell searchingmethod according to a second exemplary embodiment of the presentinvention.

As shown in FIG. 8, the cell searching device includes a receiver 810, asymbol synchronization estimation and frequency offset compensator 820,a Fourier transformer 830, and a cell ID estimator 840.

A cell searching method according to a first exemplary embodiment of thepresent invention will now be described with reference to FIG. 9.

As shown in FIG. 9, the receiver 810 receives the frame from the basestation, and the symbol synchronization estimation and frequency offsetcompensator 820 filters the received signal by the bandwidth allocatedto the synchronization channel, correlates the filtered received signaland a plurality of predetermined primary synchronization signals toacquire symbol synchronization, and estimates frequency synchronizationto compensate for a frequency offset (S910). The symbol synchronizationestimation and frequency offset compensator 820 correlates the filteredreceived signal and a plurality of predetermined primary synchronizationsignals, estimates the time having the greatest correlation value to besymbol synchronization, and transmits the number of the primarysynchronization signal having the greatest correlation value to the cellID estimator 840. In this instance, frequency offset is compensated inthe frequency domain after Fourier transform.

The Fourier transformer 830 performs a Fourier transform process on thereceived signal with reference to the symbol synchronization estimatedby the symbol synchronization estimation and frequency offsetcompensator 820 (S920).

The cell ID estimator 840 correlates the Fourier transformed receivedsignal and a plurality of predetermined secondary synchronizationsignals to estimate a cell ID group and frame synchronization (S930).The cell ID estimator 840 correlates the Fourier transformed receivedsignal and a plurality of secondary synchronization signals that aregenerated by applying P_(j,1) and P_(j,2) that are determined by theprimary synchronization signal corresponding to the number of theprimary synchronization signal transmitted by the symbol synchronizationestimation and frequency offset compensator 820 to Equation 5 toEquation 8, and estimates the frame synchronization and the cell IDgroup by using the secondary synchronization signal having the greatestcorrelation value. In this instance, when the synchronization channelsymbol in one frame is provided within one slot or one OFDM symbol,there is no need of additionally acquiring frame synchronization sincethe symbol synchronization becomes the frame synchronization.

The cell ID estimator 840 estimates the cell ID by using the number ofthe primary synchronization signal transmitted by the symbolsynchronization estimation and frequency offset compensator 820 and theestimated cell ID group (S940). In this instance, the cell ID estimator840 estimates the cell ID by referring to the mapping relation of thepredetermined primary synchronization signal number, cell ID group, andcell ID.

The estimated cell ID information can be checked by using scramblingsequence information included in the pilot symbol interval.

A cell searching method according to a second exemplary embodiment ofthe present invention will now be described with reference to FIG. 10.

The receiver 810 receives the frame from the base station, and thesymbol synchronization estimation and frequency offset compensator 820filters the received signal by the bandwidth allocated to thesynchronization channel, correlates the filtered received signal and aplurality of predetermined primary synchronization signals to acquiresymbol synchronization, and estimates frequency synchronization tocompensate the frequency offset (S710). The symbol synchronizationestimation and frequency offset compensator 820 correlates the filteredreceived signal and a plurality of predetermined primary synchronizationsignals to estimate the time having the greatest correlation value to besymbol synchronization, and transmits a plurality of correlation valuesthat are generated by correlating the primary synchronization signalsand the filtered received signal to the cell ID estimator 840. In thisinstance, the frequency offset can be compensated in the frequencydomain after Fourier transform.

The Fourier transformer 830 performs a Fourier transform process on thereceived signal with reference to the symbol synchronization estimatedby the symbol synchronization estimation and frequency offsetcompensator 820 (S720).

The cell ID estimator 840 estimates the cell ID by using a plurality ofcorrelation values transmitted by the symbol synchronization estimationand frequency offset compensator 820, the Fourier transformed receivedsignal, and correlation values of a plurality of predetermined secondarysynchronization signals (S730). The cell ID estimator 840 correlates theFourier transformed received signal and a plurality of secondarysynchronization signals that are generated by applying P_(j,1) andP_(j,2) that are determined according to the corresponding primarysynchronization signals to Equation 5 to Equation 8, and finds thesecondary synchronization signal having the greatest correlation value,regarding a plurality of respective primary synchronization signals.

The cell ID estimator 840 combines the correlation value of thecorresponding primary synchronization signal transmitted by the symbolsynchronization estimation and frequency offset compensator 820 and thecorrelation value of the secondary synchronization signal having thegreatest correlation value with the Fourier transformed received signalfrom among a plurality of secondary synchronization signals that aregenerated by applying P_(j,1) and P_(j,2) that are determined by thecorresponding primary synchronization signal to Equation 5 to Equation8, regarding a plurality of respective primary synchronization signals.

The cell ID estimator 840 estimates the frame synchronization and thecell ID group by using the secondary synchronization signal having thegreatest value generated by combining the correlation value of theprimary synchronization signal and the correlation value of thesecondary synchronization signal. The cell ID estimator 840 estimatesthe cell ID by using the estimated cell ID group and the primarysynchronization signal having the greatest value generated by combiningthe correlation value of the primary synchronization signal and thecorrelation value of the secondary synchronization signal. In thisinstance, the cell ID estimator 840 estimates the cell ID by referringto the mapping relation of the predetermined primary synchronizationsignal number, cell ID group, and cell ID.

The above-described embodiments can be realized through a program forrealizing functions corresponding to the configuration of theembodiments or a recording medium for recording the program in additionto through the above-described device and/or method, which is easilyrealized by a person skilled in the art.

While this invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A method of searching a cell by a mobile station, the methodcomprising: receiving a downlink frame including a first synchronizationsignal and two second synchronization signals; and identifying a cell byusing the first synchronization signal and at least one of the twosecond synchronization signals, wherein one of the two secondsynchronization signals includes a first short sequence scrambled with afirst scrambling sequence and a second short sequence scrambled with asecond scrambling sequence, the other of the two second synchronizationsignals includes the second short sequence scrambled with the firstscrambling sequence and the first short sequence scrambled with a thirdscrambling sequence, the first scrambling sequence is determined by thefirst synchronization signal, the second scrambling sequence isdetermined by the first short sequence, the third scrambling sequence isdetermined by the second short sequence, and the first and second shortsequences indicate cell group information.
 2. The method of claim 1,wherein in the one second synchronization signal, the first shortsequence scrambled with the first scrambling sequence and the secondshort sequence scrambled with the second scrambling sequence arealternately disposed on a plurality of subcarriers of the downlinkframe, and in the other second synchronization signal, the second shortsequence scrambled with the first scrambling sequence and the firstshort sequence scrambled with the third scrambling sequence arealternately disposed on a plurality of subcarriers of the downlinkframe.
 3. The method of claim 1, the identifying the cell comprises:identifying a cell group by using at least one of the two secondsynchronization signals; and identifying the cell in the cell group byusing the first synchronization signal.
 4. An apparatus for searching acell, the apparatus comprising: a receiving unit configured to receive adownlink frame including a first synchronization signal and two secondsynchronization signals; and a cell ID estimating unit configured toidentify a cell by using the first synchronization signal and at leastone of the second synchronization signals, wherein one of the two secondsynchronization signals includes a first short sequence scrambled with afirst scrambling sequence and a second short sequence scrambled with asecond scrambling sequence, the other of the two second synchronizationsignals includes the second short sequence scrambled with the firstscrambling sequence and the first short sequence scrambled with a thirdscrambling sequence, the first scrambling sequence is determined by thefirst synchronization signal, the second scrambling sequence isdetermined by the first short sequence, the third scrambling sequence isdetermined by the second short sequence, and the first and second shortsequences indicate cell group information.
 5. The apparatus of claim 4,wherein in the one second synchronization signal, the first shortsequence scrambled with the first scrambling sequence and the secondshort sequence scrambled with the second scrambling sequence arealternately disposed on a plurality of subcarriers of the downlinkframe, and in the other second synchronization signal, the second shortsequence scrambled with the first scrambling sequence and the firstshort sequence scrambled with the third scrambling sequence arealternately disposed on a plurality of subcarriers of the downlinkframe.
 6. The apparatus of claim 4, wherein the cell ID estimating unitidentifies a cell group by using at least one of the two secondsynchronization signals, and identifies the cell in the cell group byusing the first synchronization signal.